Cold cathode electron emission devices are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
The advent of semiconductor integrated circuit technology made possible the development and mass production of arrays of cold cathode emitters of this type. In most cases, cold cathode field emission displays comprise an array of very small conical emitters, each of which is connected to a source of negative voltage via a cathode conductor line or column. Another set of conductive lines (called gate lines) is located a short distance above the cathode lines and is orthogonally disposed relative to them, intersecting with them at the locations of the conical emitters or microtips, and connected to a source of positive voltage. Both the cathode and the gate line that relate to a particular microtip must be activated before there will be sufficient voltage to cause cold cathode emission.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and strike an electroluminescent panel that is located a short distance from the gate lines. In general, a significant number of microtips serve together as a single pixel (or subpixel) for the total display. Note that, even though the local electric field in the immediate vicinity of a microtip is in excess of 1 million volts/cm., the externally applied voltage is only of the order of 100 volts.
In FIG. 1 we show, in schematic cross-section, the basic elements of a typical cold cathode display. Metallic lines 1 are formed on the surface of an insulating substrate (not shown). Said lines are referred to as cathode columns. At regular intervals along the cathode columns, microtips 4 are formed. These are typically cones of height about one micron and base diameter about one micron and comprise molybdenum or silicon, though other materials may also be used. In many embodiments of the prior art, local ballast resistors (not shown here) may be in place between the cones and the cathode columns.
Metallic lines 3 are formed at right angles to the cathode columns, intersecting them at the locations of the microtips. A layer of insulation 2 supports lines 3, which are generally known as gate lines, placing them at the top level of the microtips, that is at the level of the apexes of the cones 4. Openings in gate lines 3, directly over emitter cavity 8 in which the microtips are located, allow streams of electrons to emerge from the tips when sufficient voltage is applied between the gate lines and the cathode columns. Because of the local high fields right at the surface of the microtips, relatively modest voltages, of the order of 100 volts are sufficient.
After emerging from the emitter cavity, electrons are further accelerated so that they strike fluorescent screen 7 where they emit visible light. Said fluorescent screen is separated from the cold cathode assembly by spacers (not shown) and the space between these two assemblies is evacuated to provide and maintain a vacuum of the order of 10 .sup.-7 torr.
It should be noted that, although the electrons are accelerated past the gate (or extraction) electrode, they are also attracted to the gate as they pass it. This arrangement (referred to as proximity focusing) leads to the formation of a diverging beam and results in a relatively large spot size at the surface of the phosphor screen. A common method of dealing with this is to add to the structure an additional, focusing, electrode similar to the gate electrode but located above it. This is illustrated in FIG. 1 where focus electrode 6 is shown as concentric with and positioned above extraction electrode 3, being separated therefrom by dielectric layer 5. The focus electrode is biased negative relative to the gate (being at or near cathode potential) so that the electron beam, as it passes through it, tends to be compressed and becomes less diverging.
There are two difficulties associated with the focus gate approach. (i) Since electrons are being repelled as they pass through it and because the effectiveness of the gate electrode is reduced by its presence, the focus gate arrangement brings about the requirement of higher gate voltages to achieve the same beam densities. This can cause a problem with breakdown in layer 2. (ii) The requirement that gate and focus electrodes must be precisely aligned relative to one another makes the manufacturing of this arrangement more prone to error and therefore more expensive. The present invention is directed to solving problem (ii)--how to guarantee perfect alignment between the gate and focus electrodes every time.
During a routine search of the prior art no references offering the same solution as the present invention were found. Several references of interest were however encountered. For example, Peng (U.S. Pat. No. 5,710,483) uses a micro-mesh to provide additional focusing of the extracted electrons. However, no attempt is made to bring about close alignment between openings in the micro-mesh and the emitter micro-tips.
Tsai (U.S. Pat. No. 5,757,138) teaches a focusing aperture that is maintained at cathode potential and including a current limiting resistor (to protect against microtip to gate shorts) which is best located electrically close to the microtip. Doan et al. (U.S. Pat. No. 5,186,670) does teach a self aligned gate and focusing ring structure. After microtip formation, several alternating layers of metal and insulation are deposited and a hole is then etched to form gate and focus openings as well as to expose the tip. This method guarantees perfect alignment between the gate and focus electrodes but has the disadvantage that misalignment between both these electrodes and the microtip is still possible.